The Prophet ﷺ said: What is unlawful by reason of consanguinity is unlawful by reason of fosterage.  

عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّ النَّبِيَّ ﷺ قَالَ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ الْوِلاَدَةِ  

Umm Salamah reported Umm Habibah said “Are you interested in my sister, Apostle of Allaah ﷺ?” He said “What should I do?” She said “You marry her” He said “Your sister?” She said “Yes”. He said “Do you like that?” she said “I am not alone with you of those who share me in this good, my sister is most to my liking. He said “She is not lawful for me.” She said “By Allaah, I was told that you were going to betroth with you Darrah to Durrah , the narrator Zuhair doubted the daughter of Abu Salamah. He said “The daughter of Umm Salamah? She said “Yes”. He said “(She is my step daughter). Even if she had not been my step daughter under my protection, she would not have been lawful for me. She is my foster niece (daughter of my brother by fosterage). Thuwaibah suckled me as well as his father (Abu Salamah). So do not present to me your daughters and your sisters.  

يَا رَسُولَ اللَّهِ هَلْ لَكَ فِي أُخْتِي قَالَ فَأَفْعَلُ مَاذَا قَالَتْ فَتَنْكِحُهَا قَالَ أُخْتَكِ قَالَتْ نَعَمْ قَالَ أَوَتُحِبِّينَ ذَاكَ قَالَتْ لَسْتُ بِمُخْلِيَةٍ بِكَ وَأَحَبُّ مَنْ شَرَكَنِي فِي خَيْرٍ أُخْتِي قَالَ فَإِنَّهَا لاَ تَحِلُّ لِي قَالَتْ فَوَاللَّهِ لَقَدْ أُخْبِرْتُ أَنَّكَ تَخْطُبُ دُرَّةَ أَوْ ذَرَّةَ شَكَّ زُهَيْرٌ بِنْتَ أَبِي سَلَمَةَ قَالَ بِنْتَ أُمِّ سَلَمَةَ قَالَتْ نَعَمْ قَالَ أَمَا وَاللَّهِ لَوْ لَمْ تَكُنْ رَبِيبَتِي فِي حِجْرِي مَا حَلَّتْ لِي إِنَّهَا ابْنَةُ أَخِي مِنَ الرَّضَاعَةِ أَرْضَعَتْنِي وَأَبَاهَا ثُوَيْبَةُ فَلاَ تَعْرِضْنَ عَلَىَّ بَنَاتِكُنَّ وَلاَ أَخَوَاتِكُنَّ